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Quelle _rwlock.py Sprache: Python |
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# Copyright Mateusz Kobos, (c) 2011
# https://code.activestate.com/recipes/577803-reader-writer-lock-with-priority-fo # released under the MIT licence import threading __author__ = "Mateusz Kobos" class RWLock: """ Read-Write locking primitive Synchronization object used in a solution of so-called second readers-writers problem. In this problem, many readers can simultaneously access a share, and a writer has an exclusive access to this share. Additionally, the following constraints should be met: 1) no reader should be kept waiting if the share is currently opened for reading unless a writer is also waiting for the share, 2) no writer should be kept waiting for the share longer than absolutely necessary. The implementation is based on [1, secs. 4.2.2, 4.2.6, 4.2.7] with a modification -- adding an additional lock (C{self.__readers_queue}) -- in accordance with [2]. Sources: [1] A.B. Downey: "The little book of semaphores", Version 2.1.5, 2008 [2] P.J. Courtois, F. Heymans, D.L. Parnas: "Concurrent Control with 'Readers' and 'Writers'", Communications of the ACM, 1971 (via [3]) [3] http://en.wikipedia.org/wiki/Readers-writers_problem """ def __init__(self): """ A lock giving an even higher priority to the writer in certain cases (see [2] for a discussion). """ self.__read_switch = _LightSwitch() self.__write_switch = _LightSwitch() self.__no_readers = threading.Lock() self.__no_writers = threading.Lock() self.__readers_queue = threading.Lock() def reader_acquire(self): self.__readers_queue.acquire() self.__no_readers.acquire() self.__read_switch.acquire(self.__no_writers) self.__no_readers.release() self.__readers_queue.release() def reader_release(self): self.__read_switch.release(self.__no_writers) def writer_acquire(self): self.__write_switch.acquire(self.__no_readers) self.__no_writers.acquire() def writer_release(self): self.__no_writers.release() self.__write_switch.release(self.__no_readers) class _LightSwitch: """An auxiliary "light switch"-like object. The first thread turns on the "switch", the last one turns it off (see [1, sec. 4.2.2] for details).""" def __init__(self): self.__counter = 0 self.__mutex = threading.Lock() def acquire(self, lock): self.__mutex.acquire() self.__counter += 1 if self.__counter == 1: lock.acquire() self.__mutex.release() def release(self, lock): self.__mutex.acquire() self.__counter -= 1 if self.__counter == 0: lock.release() self.__mutex.release() ¤ Dauer der Verarbeitung: 0.19 Sekunden (vorverarbeitet) ¤*© Formatika GbR, Deutschland |
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2026-03-28